1. Field of the Invention
The present invention relates to a method for manufacturing an organic semiconductor thin film and a semiconductor device comprising the thin film- and, more particularly, to a method for manufacturing an organic semiconductor thin film having an improved characteristic as a semiconductor and a semiconductor device comprising the thin film.
2. Description of the Background Art
A Langmuir-Blodgett's technique (hereinafter referred to as a LB method) has been conventionally well known as a method for forming an organic semiconductor thin film. The Langmuir-Blodgett's technique will be briefly described in the following. FIGS. 1A to 1C show processes for accumulating thin films on a substrate by the Langmuir-Blodgett's technique.
Referring to FIG. 1A, an appropriate liquid such as water 20 is contained in a container. Thereafter, a material of a monomolecular film 23 is dissolved by an organic solvent to provide an organic solution. The organic solution is dropped in the water 20. The organic solution is spread over the surface of the water 20. Thereafter, the organic solvent is volatilized, leaving a monomolecular film 23 on the surface of the water 20.
Referring to FIG. 1B, as the substrate 21 is gradually lowered, the monomolecular film 23 is attached to the substrate 21. Thereafter, when the substrate 21 is left from the water 20, a substrate 21 having the monomolecular film 23 attached thereon is provided. By repeating the same operation, a substrate 21 having several to several ten layers of monomolecular films 23 accumulated thereon is provided, as shown in FIG. 1C.
For example, Japanese Patent Laying-Open No. 141246/1983 discloses technique of forming a phthalocyanine organic semiconductor thin film by the LB method from a tetra-tert-butyl phthalocyanine compound having four tert-butyl groups introduced in a molecule as shown in FIG. 2. In addition, another technique of composing the organic semiconductor phthalocyanine thin film by the LB method from the phthalocyanine compound having substituent groups (--CH.sub.2 NHC.sub.3 H.sub.7) introduced in non-symmetrical positions in the molecule, as shown in FIG. 3 is disclosed in the literature (Thin Solid Films, 132, 113 (1985).
The above described substituent group is introduced to make phthalocyanine compound soluble in the organic solvent. The phthalocyanine compound is insoluble in the organic solvent unless such substituent group exists.
However, referring to FIG. 4A, t-butyl group has a large volume 25 occupying the space, since the t-butyl group comprises three methyl groups coupled to a tertiary carbon atom. That is the reason why t-butyl group is bulky.
When such a Pc compound having bulky substituent group is used for forming an organic semiconductor thin film in accordance with the above described LB method, the stacking of the cumulative layers is not sufficient as shown in FIG. 5A, so that t-butyl group 27 serves as an insulating layer, preventing effective use of the characteristic of the semiconductor.
In case of the phthalocyanine compound shown in FIG. 3, the substituent group (--CH.sub.2 NHC.sub.3 H.sub.7) is introduced at non-symmetrical positions of the molecule. When an organic semiconductor thin film is formed by using such phthalocyanine compound in accordance with the LB method, packing of the cumulative layer is not sufficient. Consequently, the substituent group serves as an insulating layer, preventing effective use of the characteristics of the semiconductor.